Dolphins produce ultrasound. Dolphins and dolphin therapy: benefits and treatment
ELECTRON BEAM- a flow of electrons moving along close trajectories in one direction, having dimensions significantly larger in the direction of movement than in the transverse plane. Since E. p. is a collection of charges of the same name. particles, inside it there is space charge electrons, creating their own. electric field. On the other hand, electrons moving along similar trajectories can be considered as linear currents that create their own. mag. field. Electric field of spaces.
creates a force tending to expand the beam (“Coulomb repulsion”), mag. the field of linear currents creates a Lorentz force that tends to compress the beam. The calculation shows that the action of spaces. charge begins to have a noticeable effect (at electron energies of several keV) at currents of several. tenths of mA, while the “contracting” action of its own. mag. field is noticeably manifested only at electron velocities close to the speed of light - electron energy of the order of MeV. Therefore, when considering E. items used in dep. electronic devices, technical installations, first of all it is necessary to take into account the effect of its own. spaces. charge, and the action of its own. mag. fields are taken into account only for relativistic beams. E. p intensity
. Basic The criterion for the conditional division of electrical energy into non-intensive and intensive ones is the need to take into account the action of the field of its own. spaces. charge of the beam electrons. Obviously, the greater the beam current, the more spaces there are. charge, stronger repulsion. On the other hand, the higher the speed of the electrons, the less it will affect the nature of the movement of the electrons. electric beam field - the higher the electron energy, the “harder” the beam. Quantitative action of the field of spaces. charge is characterized by a coefficient. space charge - perv ean s o m, defined as Where I -beam current; U -accelerating voltage that determines energy.
electron beam Noticeable influence of spaces. charge on the movement of electrons in the beam begins to appear when P>=P* =.
= 10 -8 A/V 3/2 = 10 -2 µA/V 3/2. Therefore, it is customary to refer to intense beams as electron beams with<Р*
) small cross-section, often called electron beams, calculated according to the laws of geom. electronic optics without taking into account the action of the intrinsic field. spaces. charge, are formed using electronic spotlights and are used mainly in various..
electron beam devices In intense beams the action of intrinsic spaces. charge significantly affects the characteristics of electrical energy. Firstly, intense electrical energy in a space free from external influences. electric and mag. fields, due to Coulomb repulsion it expands indefinitely; secondly, due to denial. electric As the electron charge in the beam increases, the potential in the beam drops. If using external electric or mag. fields to limit the expansion of an intense beam, then with a sufficiently large current, the potential inside the beam can drop to zero, and the beam will “break off”. Therefore, for intensive beams there is a concept of limiting (maximum) perveance. Practically, when limiting beam expansion, ext. fields, it is possible to form extended stable intense beams with P
5 . 10 µA/V 3/2.
Complete math. Description of intense electron beams is difficult, since a real electron flow consists of many moving electrons, and it is almost impossible to take into account the interaction between them. By introducing certain simplifying assumptions, in particular, replacing the sum of forces acting on a selected electron from neighboring electrons with the force of action on this electron by a certain electrically charged medium with a continuously distributed spatial density. charge and breaking the entire beam into a set of “current tubes”, it is possible to calculate with the help of a computer with sufficient for practical purposes. goals accuracy main. parameters of an intense beam: beam shape (envelope), distribution of current density and potential over the beam cross section. Geometry E. p . In practice, beams of three configurations are used: tape (flat), having the shape of a rectangle in cross section with a “thickness” much smaller than the “width”, axisymmetric, having the shape of a circle in cross section, and tubular, having the shape of a ring in cross section. For the formation of electrical energy of such types, appropriate electron guns
and restriction systems.
Radial component of electric voltage. field at the boundary of an axisymmetric beam is directly proportional to the beam current and inversely proportional to the radius of its cross section and the speed of the beam electrons. This creates a force directed away from the axis, tending to expand the beam. The larger the current, the smaller the speed and radius of the beam, the greater the pushing force. Theoretically, in axisymmetric beams, electron trajectories cannot cross the axis, and the beam cross-section cannot be reduced to a point, since as the cross-section decreases, the repulsive force increases indefinitely.
Envelopes of axisymmetric electron beams: g 0 -the angle of entry of the beam into the field-free region is simpleearlyness; r 0 - initial radius; 1 - divergent beam (g 0 >0); 2-cylindrical beam (g 0 =0); 3, 4, 5-converging bundles (g 0<0). Пучок 4 - опти small, since the crossover (smallest cross section) the beam is at the farthest distance (z/ l=0.5) from the original plane.
Envelope of an intense axisymmetric beam in a space free from electricity. and mag. fields, is described by a dependence close to exponential. In Fig. shows the envelopes of axisymmetric beams that have a cylindrical (curve 2, g 0 = 0), divergent (curve 1, g 0 >0) and convergent (curves 3-4, g 0) before entering free space<0)
формы (g 0 - угол наклона касательной к огибающей пучка, угол
входа). Как видно на рис., пучки, первоначально сформированные как цилиндрические
(g 0 = 0) и расходящиеся (g 0 >0), expand indefinitely in field-free space; bundles formed as converging ones are initially compressed ( r/r 0 <1), проходят плоскость наименьшего
сечения (плоскость кроссовера), затем также начинают расширяться. Радиус мин.
сечения пучка - радиус кроссовера-определяется выражением
. Basic The criterion for the conditional division of electrical energy into non-intensive and intensive ones is the need to take into account the action of the field of its own. spaces. charge of the beam electrons. Obviously, the greater the beam current, the more spaces there are. charge, stronger repulsion. On the other hand, the higher the speed of the electrons, the less it will affect the nature of the movement of the electrons. electric beam field - the higher the electron energy, the “harder” the beam. Quantitative action of the field of spaces. charge is characterized by a coefficient. space charge - perv ean s o m, defined as r 0 is the radius of the EP before entering free space.
The smaller the perveance and the larger | g 0 |. With an increase (in absolute value) of the angle of entry of the beam into the field-free space (g 0), the crossover plane first moves away from the original plane,
thus begins to approach it (sequentially curves 3, 4, 5). For each value of the perveance, there is an optimal “angle of approach” g 0, at which the crossover is at its maximum. is removed from the original plane, that is, an electron beam with a given perveance can be drawn to the greatest distance with a radius not exceeding the original one.
Intensive tape beams in a free-from-electricity environment. and mag. The fields in space also expand indefinitely (become “thicker”), and the contour of the beam envelope is described by a parabolic.
Let's experiment. verification of the obtained calculated relationships is difficult, since the very concept of the boundary (envelope) of an intense beam is conditional, since in real beams the current density when moving away from the axisymmetric axis or from the sr. the plane of the ribbon beams decreases gradually, and the boundary of the beam is conventionally considered to be a circle or a straight line, along which the current density is a certain small fraction (~0.1) of its maximum. values on the axis.
Potential E. p. The potential drop inside the intense beam limits the possibility of forming an extended intense beam with high perveance. Theoretical Research shows that in an intense unlimited flow that fills the space between two flat parallel conducting surfaces with the same potential, which determines the energy of the flow electrons, with increasing current in avg. plane, a minimum potential is formed. Upon reaching P= 18.64 µA/V 3/2 potential drops to zero, a virtual cathode,Some of the electrons pass through the minimum plane, some are reflected to the original plane, and the normal current flow is disrupted. Let's experiment. the check confirms this, precisely when approaching In intense beams the action of intrinsic spaces. charge significantly affects the characteristics of electrical energy. Firstly, intense electrical energy in a space free from external influences. electric and mag. fields, due to Coulomb repulsion it expands indefinitely; secondly, due to denial. electric As the electron charge in the beam increases, the potential in the beam drops. If using external electric or mag. fields to limit the expansion of an intense beam, then with a sufficiently large current, the potential inside the beam can drop to zero, and the beam will “break off”. Therefore, for intensive beams there is a concept of limiting (maximum) perveance. Practically, when limiting beam expansion, ext. fields, it is possible to form extended stable intense beams with to 18.64 μA/V 3/2, instabilities appear in the flow of the electronic layers, and the passage of current is disrupted.
In real E. p., limited externally. electric and mag. fields, a drop in potential also occurs, but since in most devices that use intense electron beams, an extended beam is passed through a pipe with a positive voltage. potential, it is possible to maintain a potential on the surface of the bundle close to the potential of the pipe. But even in the presence of a conducting pipe, the potential on the axis is axisymmetric or in cf. the plane of the ribbon beam decreases noticeably, and upon reaching a sufficiently large perveance (greater than in the case of an unbounded flow), instability arises and the beam breaks off.
Formation of E. p. Since the electronic space in free space expands without limit, for practical purposes. When using intense beams, in addition to the system that forms the beam—an electron gun—a system is required that limits the beam divergence. The expansion of E. p. is limited with the help of external. electric and mag. fields. Classic an example of an extended intensive e.p.-t.n. FLOW OF BRILL LUEN - cylindrical. a beam limited by a longitudinal homogeneous magnetic field. field. When defined the ratio of four quantities - beginning. radius r 0 , beam current Where, -beam current; 0, which determines the energy of electrons before entering the magnet. field, and magnetic induction of longitudinal homogeneous magnetic field. fields B 0 - it is theoretically possible to obtain a stable cylindrical. E.p. At the optimal ratio r 0 ,
Where, -beam current; 0 and B 0 max. The perveance of the Brillouin flux reaches 25.4 μA/V 3/2. At max. The perveance potential at the beam axis is only 1/3 of the value at the boundary. With limited magnetic With the field of tubular beams, even larger perveance values can be obtained.
In practice, it is not possible to form extended EPs with a perveance close to the theoretically maximum possible due to a number of reasons: the scatter of the beginning. speeds of electrons emitted by the cathode, difficulties in creating limiting fields of a strictly specified configuration, practical. the inability to strictly fulfill the beginning. conditions for introducing the beam into the limiting system, etc. Real electron beams have wavy and pulsating boundaries, and the shape of the beam does not remain unchanged. Therefore, to prevent the beam electrons from settling on the surface of the flight channel, the radius of the conductive tube through which an intense beam is passed is selected to be 20-30% larger than the beam radius.
Lit.: Alyamovsky I.V., Electron beams and electron guns, M., 1966; Molokovsky S.I., Sushkov A.D., Intense electron and ion beams, 2nd ed., M., 1991.
A. A. Zhigarev.
>>Physics: Electron beams. Cathode-ray tube
If a hole is made in the anode of an electron tube, then some of the electrons accelerated by the electric field will fly into this hole, forming an electron beam behind the anode. The number of electrons in the beam can be controlled by placing an additional electrode between the cathode and anode and changing its potential.
Properties of electron beams and their applications. An electron beam hitting bodies causes them to heat up. IN modern technology This property is used for electronic melting in vacuum of ultrapure metals.
When fast electrons hitting a substance are slowed down, a x-ray radiation. This phenomenon is used in X-ray tubes.
Some substances (glass, zinc and cadmium sulfides) when bombarded by electrons glow. Currently, materials of this type (luminophores) are those in which up to 25% of the energy of the electron beam is converted into light energy.
Electron beams are deflected by an electric field. For example, passing between the plates of a capacitor, electrons are deflected from a negatively charged plate to a positively charged one ( Fig. 16.20).
The electron beam is also deflected in magnetic field
. Flying over north pole magnet, electrons are deflected to the left, and flying over the southern one, they are deflected to the right ( Fig. 16.21). The deviation of electron flows coming from the Sun in the Earth's magnetic field leads to the glow of gases in the upper layers of the atmosphere ( Polar Lights) is observed only at the poles.
The ability to control an electron beam using an electric or magnetic field and the glow of a phosphor-coated screen under the action of the beam is used in a cathode ray tube.
A cathode ray tube is the main element of one of the types of televisions and an oscilloscope - a device for studying rapidly changing processes in electrical circuits (Fig. 16.22).
The structure of a cathode ray tube is shown in Figure 16.23. This tube is a vacuum cylinder, one of the walls of which serves as a screen. A source of fast electrons is placed at the narrow end of the tube - electron gun (Fig. 16.24). It consists of a cathode, a control electrode and an anode (usually several anodes are located one behind the other). Electrons are emitted by the heated oxide layer from the end of the cylindrical cathode WITH, surrounded by a heat shield N. They then pass through a hole in the cylindrical control electrode IN(it regulates the number of electrons in the beam). Each anode ( A 1 And A 2) consists of disks with small holes. These discs are inserted into metal cylinders. A potential difference of hundreds and even thousands of volts is created between the first anode and the cathode. A strong electric field accelerates electrons, and they acquire greater speed. The shape, location and potentials of the anodes are chosen so that, along with the acceleration of electrons, the electron beam is also focused, i.e., the cross-sectional area of the beam on the screen is reduced to almost point sizes.
On its way to the screen, the beam sequentially passes between two pairs of control plates, plate-like flat capacitor (see Fig. 16.23). If there is no electric field between the plates, then the beam is not deflected and the luminous point is located in the center of the screen. When a potential difference is imparted to vertically located plates, the beam is displaced in the horizontal direction, and when a potential difference is communicated to horizontal plates, it is displaced in the vertical direction.
The simultaneous use of two pairs of plates allows you to move the luminous point across the screen in any direction. Since the mass of electrons is very small, they almost instantly, i.e., within a very a short time, respond to changes in the potential difference of the control plates.
In a cathode ray tube used in a television (the so-called kinescope), the beam created by the electron gun is controlled using a magnetic field. This field is created by coils placed on the neck of the tube ( Fig. 16.25).
The color kinescope contains three spaced electron guns and a screen with a mosaic structure, composed of three types of phosphors (red, blue and green). Each electron beam excites phosphors of one type, the glow of which together creates a color image on the screen.
Cathode ray tubes are widely used in displays- devices connected to electronic computers (computers). The display screen, similar to a television screen, receives information recorded and processed computer. You can directly see text in any language, graphics various processes, images of real objects, as well as imaginary objects that obey the laws written in the program computer.
In cathode ray tubes, narrow electron beams are formed, controlled by electrical and magnetic fields. These beams are used in oscilloscopes, television picture tubes, and computer displays.
???
1. How are electron beams controlled?
2. How does a cathode ray tube work?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10
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Beams of electrons moving from high speeds, can be used to produce x-rays, melt and cut metals. The ability of electron beams to be deflected by electric and magnetic fields and cause crystals to glow is used in cathode ray tubes.
Electron beams are produced using an electron gun - a vacuum device, usually a diode, in which electrons fly out of the cathode thanks to Ch. The beams are focused by electronic lenses, which create the necessary electrical energy.
Beta rays are beams of electrons. The zero index reflects the fact that the electron mass is negligible compared to the nucleon mass. Index - 1 indicates that the particle in question has a negative sign, equal in magnitude but opposite in sign to the charge of the proton.
UV irradiation or an electron beam (initiating agent) initiates a fast molecular radical reaction, releasing the energy stored in the mixture in the form of a short pulse of coherent radiation.
Therefore, electric fields with a continuous change in potential are used to influence electron beams.
It should be noted that electron beams interact strongly with matter. The maximum permissible sample thickness is only a few microns. This circumstance significantly limits the capabilities of the method for studying liquid disperse systems. Typically, fine-crystalline samples deposited on specially treated substrates are studied.
Therefore, it turns out to be possible to transmit to a beam of electrons flying along the o: n cis. A beam of electrons, interacting with this field, can transfer part of its energy to the line and thereby amplify the waves traveling in the line, or excite such waves.
In an ordinary, unpolarized beam of electrons or positrons, the spins of the particles are directed randomly. Thus, after some time (relaxation time), an ordinary beam of electrons or positrons becomes polarized - the spins of the particles take on an ordered orientation.
Such waves can be excited by longitudinal beams of electrons or ions. As for the waves propagating in the direction of electron drift (a 0), then for their growth in time only the presence of a density gradient is sufficient.
Polymer chains are directly cross-linked by high-energy electron beams. These electrons generate PE macroradicals, extracting hydrogen radicals. Typically this method is used for the manufacture of 1 1 kV cables with XLPE insulation.
Electrostatic cathode electron lens. / - cathode. 2 - focusing electrode. 3-anode Thin lines are equipotentials. O is one of the cathode points. Shaded space-section of the region occupied by the flow of electrons emitted by point O.| Electrostatic cylindrical electronic lenses. a-diaphragm with a slit. b-immersion lens consisting of two plates. In the region of passage of charged particles, the lens field does not change in the direction parallel to the diaphragm slits or the gaps between the plates of adjacent electrodes.| Section of the electrodes of electrostatic cylindrical lenses by a plane passing through the z axis perpendicular to the middle plane. a-cylindrical (slit diaphragm. b-immersion cylindrical lens. - single cylindrical lens. g-cathode cylindrical lens. K, and K2 are the potentials of the corresponding electrodes. electron beam. / - electrodes. 2-magnetic pole. | Doublet of two quadrupole electrostatic lenses. |